Various light sources, particularly, laser light sources with a variable oscillation wavelength, have been used in the field of communication networks and in the field of inspection apparatuses.
In the field of communication networks, the demand for high-speed wavelength switching is growing. In the field of inspection apparatuses, the demand for high-speed and wide-range wavelength sweeping, etc., is growing.
Wavelength-variable (swept) light sources in inspection apparatuses have applications in laser spectrometers, dispersion measuring apparatuses, film thickness measuring apparatuses, swept source optical coherence tomography (SS-OCT) apparatuses, and the like.
Optical coherence tomography is an imaging technique which is designed to obtain tomographic images of a specimen using optical interference and which has recently been actively studied in the medical field in order to realize, for example, micron-order spatial resolution and noninvasiveness.
In swept source optical coherence tomography, depth information is obtained using spectral interference without using spectrometers, resulting in low loss in light intensity. Swept source optical coherence tomography is also expected to acquire a high SN ratio image.
Here, in an apparatus that produces an image using interference of light emitted from a light source, such as an optical coherence tomography apparatus, the spectrum of the light emitted from the light source affects an image to be produced, and the image may be affected by the spectral shape.
Under such circumstances, PTL 1 discloses a technique for multiple relay transmission (long-distance relay transmission) using an optical amplification apparatus, in which a reduction in signal bandwidth and the like which are caused by periodical ripples in gain due to reflection in the optical amplification apparatus is prevented. Specifically, it is disclosed that the frequency at which ripples are created in a plurality of optical amplifiers is controlled to cancel the ripples in the respective optical amplifiers.